1. Field of the Invention
The present invention relates to hydraulic torque transmitting couplings and more particularly to control couplings of the rotational speed differential responsive type used as a driving torque distributing device, a differential slip limiting device, etc. in a multi-wheel drive vehicle such as a four-wheel drive vehicle.
2. Description of the Prior Art
An example of a prior art rotational speed differential responsive type control coupling is disclosed in Japanese Patent Provisional Publication No. 63-62635 or 63-62636.
A problem of the prior art control coupling is that it is liable to be heated up to an excessively high temperature when a vehicle on which it is installs makes a continuous running through an icy, muddy or sandy terrain or the like low-.mu. place, thus causing deterioration of parts made of rubber, sythetic resin, etc. and also deterioration of oil within a short period of usage and therefore considerably shortening the life of the coupling.